This is an especially grievous act if the reply is just a couple of lines, followed by a 200-line exact copy of the message that we all got five minutes ago, complete with mail signatures, mailing list footer stuff, Yahoo ads, and, often as not, multiple generations of embedded responses.
Of course, it's only partly the morons' fault. It's also the fault of the developers of the lowest-common-denominator e-mail clients that make it easiest to just leave the whole damn multi-generation reply thread after your five-word response.
You're all clever and polite enough to trim off the entire text of the previous message before sending your reply to a list, right? After all, we've all already seen it.